The present invention relates to switching voltage regulators. More specifically, it relates to stability control of a switching voltage regulator.
Pulse width modulated (PWM) voltage regulators are used to convert power from one voltage level to a second voltage level. A Buck voltage regulator typically operates by switching from a first voltage level to a second voltage level that is lower than the first voltage level. FIG. 1 illustrates an embodiment of a Buck regulator topology. Another regulator topology is used to obtain a boost converter that boosts the first voltage level to a second voltage level that is higher than the first voltage level. Still another topology is used to obtain a Buck-boost regulator that may convert the first voltage level to a second voltage level that is either higher or lower than the first voltage level. See Mitchell, DC-DC Switching Regulator Analysis, Bloom Associates, 1999, herein incorporated by reference in its entirety, for further background information regarding switching regulator design.
Switching regulators are highly efficient at converting from one voltage power level to another voltage power level. However, as described in Mitchell, switching regulators also require careful circuit design to obtain stability. One approach to stability is tuning of the circuit by a technician, which raises the cost of the product. Another approach is to specify external components having narrow tolerances, which leads to higher component cost. Also, the characteristics of components can change over time due to aging resulting in degradation of the stability of the converter circuit.
An embodiment of a control circuit for a switching voltage regulator circuit, according to the present invention, includes a measuring circuit having a first input terminal coupled to an output terminal of the switching voltage regulator circuit and a second input terminal coupled to an output of a switch drive circuit of the switching voltage regulator circuit. The measuring circuit is configured to generate a phase difference signal responsive to an output voltage signal at the output terminal of the switching voltage regulator circuit and an output signal of the switch drive circuit. The control circuit also includes a transfer function control circuit having a first input terminal for receiving the phase difference signal, the transfer function control circuit being configured to generate a transfer function control signal responsive to the phase difference signal such that the transfer function control signal causes at least one of a first zero or pole to be introduced to a transfer function of a voltage control feedback loop of the switching voltage regulator circuit. A variable characteristic element is disposed in the current control feedback loop and has a variable characteristic that varies responsive to the transfer function control signal. In a further refinement of this embodiment, the measuring circuit includes a first strip section having an input terminal coupled to the first input terminal of the measuring circuit, where the first strip section is configured to generate a measured phase signal in response to the output voltage signal. The measuring circuit also includes a second strip section having an input terminal coupled to the second input terminal of the measuring circuit, where the second strip section is configured to generate a reference signal responsive to a signal at the output of the switch drive circuit. A multiplier receives and multiplies the measured phase signal and the reference signal to generate the phase difference signal and a low pass filter filters the phase difference signal.
An embodiment of a method, according to the present invention, for automatically adjusting a switching voltage regulator circuit to account for an external component calls for measuring a phase of a ripple signal caused by the external component, generating a reference signal, and comparing the measured phase of the ripple signal to the reference signal to obtain a phase difference signal. The method then calls for adjusting a transfer function of a feedback path of the switching voltage regulator responsive to the phase difference signal to obtain stable operation of the switching voltage regulator circuit. In a further refinement of this embodiment, the step of adjusting a transfer function of a feedback path further includes converting the phase difference signal to a variable characteristic control signal and adjusting a variable characteristic in the feedback path of the switching voltage regulator responsive to the variable characteristic control signal. In still another refinement of this embodiment, the step of adjusting a variable characteristic in the feedback path of the switching voltage regulator responsive to the variable characteristic control signal further comprises adjusting a variable capacitance in the feedback path responsive to the variable characteristic control signal. Another embodiment of the method further calls for measuring an amplitude of the ripple signal. This embodiment then calls for converting the phase difference signal and the measured amplitude of the ripple signal into a second variable characteristic control signal and adjusting the transfer function of the feedback path of the switching voltage regulator responsive to the second variable characteristic control signal to improve a transient response of the switching voltage regulator circuit. In a further refinement of this embodiment, the step of adjusting the transfer function of the feedback path of the switching voltage regulator responsive to the second variable characteristic control signal to improve a transient response of the switching voltage regulator circuit further includes adjusting a gain in the feedback loop responsive to the second variable control signal and introducing at least one of a second pole or zero to the transfer function responsive to the second variable control signal.